Disclosed in embodiments herein are methods and systems for improving the quality of image rendering in digital printing systems and more particularly to image processing which differentiates between lines and edges of thicker lines and other image objects, and processes them differently in corresponding line width control and edge growth control operations for improved rendering.
Many images include relatively thin line image objects. For example, some characters of text are considered to be made up of thin lines and curves. Additionally, many business forms include lines and grids for separating portions of a document. It is desirable to be able to adjust images in order to enhance or improve their appearance. For instance, it may be desirable to adjust the thickness of lines in an image. For example, one way to adjust the darkness or lightness or enhance the contrast of a document being printed or photocopied is to adjust the thickness of lines.
Additionally, it may be desirable to adjust the thickness of lines in an image in order to compensate for a drift or imperfection in an image rendering system. For instance, an ink or toner may spread or be absorbed by a print media more or less readily than anticipated. If an ink spreads more readily than an ideal or anticipated amount, then, for example, a white line or feature printed on a black, gray or colored background, might appear thinner than desired or may be completely filled in by spreading colorant from neighboring portions of an image. If an ink or toner or other colorant spreads less readily than anticipated, then a black, gray or colored line may appear thinner than desired. By adjusting a digital representation of the image or selected image objects of the image to be larger or thicker than originally called for, it is possible to compensate for such variations in colorant and print media behavior.
Moving an edge or modifying a line width may also be performed to change features in an image, such as lengthening typographic serifs, and sharpening corners, as taught by U.S. Pat. No. 6,775,410, entitled “Image Processing Method for Sharpening Corners of Text and Line Art,” by R. Loce, et al., and US Publication No. US20050129328A1, entitled “Corner Sharpening Of Text And Line Art In A Super Resolution Anti-Aliasing Image Path”, by E. Saber, et al., which are hereby incorporated herein by reference in their entirety.
Some current line width control (LWC) operations, such as those taught by US 2006/0285764 A1, “Model-Based Line Width Control,” B. Xu, R. Loce, S. Schweid, which is hereby incorporated by reference herein in its entirety, can be used to determine if a line is detected within one or more of the pixel group windows for processing to achieve a desired effect. The range of controllable lines is limited by the size of the observation window. What is needed is a low complexity real-time method for modifying the growth of edges of thicker lines that can operate with an affordable window size.
Digital “halftoning” refers to encoding methods that are used to reduce the number of quantization levels per pixel in a digital image, while maintaining the gray appearance of the image at normal viewing distance. Halftoning is widely employed in the printing and displaying of digital images. The need for halftoning encoding arises either because the physical processes involved are binary in nature or the processes have been restricted to binary operation for reasons of cost, speed, memory or stability in the presence of process fluctuations. Examples of such processes are most printing presses, ink jet printers, binary cathode ray tube (CRT) displays, and laser xerography. In most printing and displaying applications, the halftoned image is composed ideally of two gray levels, black and white. Spatial integration, plus higher level processing performed by the human visual system, of local area coverage of black and white pixels, provides the appearance of a gray level, or “continuous tone”, image. Many halftone techniques readily extend to color and to quantization using more than two levels.
Line width adjustment is also performed in halftone printing systems. Many consumers have indicated a greater preference for halftone print quality than for the quality of printed line art. Efforts are therefore being made to further reduce noise and instability of halftones using optimized marking process parameters. These same parameters can affect printed widths of lines, and also affect the printed widths differently based on the digital width of the line. It is therefore desirable to control lines of different widths by different amounts using methods other than marking process parameters.